1. Field of the Invention
The present invention relates in general to digital communication systems and in particular to mobile radio communication systems. Still more particularly, the invention relates to a method of reception and a receiver in a mobile radio system operating in a single antenna interference cancellation environment.
2. Description of the Related Art
The most widespread standard in cellular wireless communications is currently the Global System for Mobile Communications (GSM). GSM employs a combination of Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) for the purpose of sharing the spectrum resource. GSM networks typically operate in the 900 MHz and 1900 MHz frequency ranges. For example, GSM-900 commonly uses radio spectrum in the 890-915 MHz bands for the uplink (Mobile Station to Base Transceiver Station) and in the 935-960 MHz bands for the downlink (base station to mobile station), providing 124 RF channels spaced at 200 kHz, and GSM-1900 uses the 1850-1910 MHz bands for the uplink and 1930- 1990 MHz bands for the downlink. The spectrum for both uplink and downlink is divided into 200 kHz-wide carrier frequencies using FDMA, and each base station is assigned one or more carrier frequencies. Each carrier frequency is divided into eight time slots using TDMA. Eight consecutive time slots form one TDMA frame, with a duration of 4.615 ms. A physical channel occupies one time slot within a TDMA frame. Each time slot within a frame is also referred to as a burst. TDMA frames of a particular carrier frequency are numbered, and formed in groups of 26 or 51 TDMA frames called multi-frames.
GSM systems typically employ one or more modulation schemes to communicate information such as voice, data, and/or control information. These modulation schemes may include GMSK (Gaussian Minimum Shift Keying), M-ary QAM (Quadrature Amplitude Modulation) or M-ary PSK (Phase Shift Keying), where M=2n, with n being the number of bits encoded within a symbol period for a specified modulation scheme. The most common modulation scheme, GMSK, is a constant envelope binary modulation scheme allowing raw transmission at a maximum rate of 270.83 kilobits per second (Kbps).
Wireless communication systems are placing an ever-increasing demand on capacity to transfer both voice and data services. While GSM is efficient for standard voice services, high-fidelity audio and data services demand higher data throughput rates. The General Packet Radio Service (GPRS), EDGE (Enhanced Data rates for GSM Evolution) and UMTS (Universal Mobile Telecommunications System) standards have been adopted to increase capacity in GSM systems.
General Packet Radio Service (GPRS) is a non-voice service that allows information to be sent and received across a mobile telephone network. It supplements Circuit Switched Data (CSD) and Short Message Service (SMS). GPRS employs the same modulation schemes as GSM, but higher data throughput rates are achievable with GPRS since it allows for an entire frame (all eight time slots) to be used by a single mobile station at the same time.
EDGE (Enhanced Data rates for GSM Evolution) and the associated packet service EGPRS (Enhanced General Packet Radio Service) have been defined as a transitional standard between the GSM/GPRS (Global System for Mobile Communications/General Packet Radio Service) and UMTS (Universal Mobile Telecommunications System) mobile radio standards. Both GMSK modulation and 8-PSK modulation are used in the EDGE standard, and the modulation type can be changed from burst to burst. GMSK is a non-linear, Gaussian-pulse-shaped frequency modulation, and 8-PSK modulation in EDGE is a linear, 8-level phase modulation with 3π/8 rotation. However, the specific GMSK modulation used in GSM can be approximated with a linear modulation (i.e., 2-level phase modulation with a π/2 rotation). The symbol pulse of the approximated GSMK and the symbol pulse of 8-PSK are identical.
It is well known that the major source of noise and interference experienced by GSM communication devices operating in typical cellular system layouts (supporting a non-trivial number of users) is due to co-channel or adjacent channel interference. Such noise sources arise from nearby devices transmitting on or near the same channel, as in frequency reuse, or from adjacent channel interference due to spectral leakage, for example. Frequency reuse is a method to increase system capacity by increasing the frequency reuse factor, whereby the communications system allocates the same frequency to multiple cellular sites in closer proximity. Unfortunately, signals intentionally introduced by frequency reuse and other stray signals can interfere with the proper transmission and reception of voice and data signals and can lower system capacity. Additionally, even where no other signal interference is present, the received signal may consist of multiple copies of the transmitted data sequence due to multi-path channel conditions, for example (This effect is sometimes referred to as self-interference). Accordingly, a receiver must be capable of processing and extracting desired information from a signal with strong interference from frequency reuse to retain the advantages of increased capacity.
Traditionally, interference cancellation techniques have had limited success focusing on adjacent channel interference by using several filtering operations to suppress frequencies of the received signal not occupied by the desired signal. Correspondingly, co-channel interference techniques have been proposed, such as joint demodulation, which generally require joint channel estimation methods to provide a joint determination of the desired and co-channel interfering signal channel impulse responses. Given known training sequences, all the co-channel interference can be estimated jointly, however, this joint demodulation requires a large amount of processing power, which constrains the number of equalization parameters that can be used efficiently.
A recently proposed standard for advanced communications systems and receiver algorithms called Single Antenna Interference Cancellation (SAIC) is designed for the purpose of improving system capacity through increasing frequency reuse. SAIC enhances single-antenna receiver performance in the presence of co-channel interference resulting from increased frequency reuse. Current SAIC receiver algorithms are generally optimized for GMSK modulated signals, since gains of SAIC tend to be smaller for 8-PSK modulated signals. In an SAIC operational environment, GMSK traffic on neighboring cells is permitted to reuse common frequencies, thereby significantly increasing network bandwidth, while still being able to tolerate significantly higher co-channel and multi-channel interference than is accommodated by conventional GMSK/EDGE environments. The SAIC operation environment, as defined herein, pertains to (a) the actual physical environment in which the system must operate, which in this case is well encapsulated in the mobile/wireless channel model (plus operating band), and (b) the application and service under consideration as particular attributes.
Frequency error in the received signal includes the Doppler frequency shift over the channel and frequency offset introduced by the local oscillator (LO) within the receiver. For example, with a 1900 MHZ carrier, the maximum Doppler shift for TU50 (as defined in GSM) can be as much as 87 Hz. This frequency error is found to significantly impact the SAIC receiver performance if not corrected. Frequency error estimation is required to be made on a burst-by-burst basis, which the receiver uses to tune it's LO. One of the SAIC solutions is the SAIC linear equalizer. Different from the conventional linear equalizer, SAIC linear equalizer includes two real FIR (finite impulse response) filters, each separately applied to the real and imaginary components of the ½π de-rotated received signal. The two FIR filter outputs are combined into a real-valued signal at the output of the SAIC linear equalizer. Different from a conventional equalizer, the output has real and imaginary components. This framework of the SAIC linear equalizer is unable to provide capacity for frequency error information, and consequently creates difficulty for frequency error estimation and correction. What is needed is an efficient frequency estimation and correction methodology associated with the SAIC linear equalizer that provides superior frequency error estimation and correction in a high interference SAIC operational environment.
In the accompanying drawings, elements might not be to scale and may be shown in generalized or schematic form or may be identified solely by name or another commercial designation.